A. C. motor control apparatus and method

ABSTRACT

In apparatus for controlling the speed and torque of a multiphase a.c. motor in accordance with digital control signals, a first regulation circuit regulates the frequency of the a.c. electrical signal applied to the motor and another regulation circuit regulates the voltage of the a.c. signal. Both regulation circuits are controlled by digital control signals in a series of discrete steps over a range of control values and the control signals are supplied periodically to the regulation circuits. The speed and torque are controlled in accordance with an algorithm relating speed and torque to the frequency and voltage of the source of electric power which is applied to the motor, calculated by computing apparatus on a periodic basis. In one embodiment the position of the magnetic flux of the stator is controlled directly.

United States Patent [191 Kirkham et al.

[54] A. C. MOTOR CONTROL APPARATUS AND METHOD [75] Inventors: Edward E. Kirkham, Brookfield;

Frank Zankl, Milwaukee; Richard E. Stobbe, Greenfield; John J. Schachte, Wauwatosa, all of Wis.

[73] Assignee: Kearney & Trecker Corporation,

Milwaukee, Wis.

[22] Filed: Sept. 1, 1972 [21] Appl. N0.: 285,813

[4 1 Apr. 15, 1975 Primary ExaminerRobert K. Schaefer Assistant Examiner-John J. Feldhaus Attorney, Agent, or Firm-Hi1l, Gross, Simpson, Van Santen, Steadman, Chiara & Simpson [57] ABSTRACT In apparatus for controlling the speed and torque of a multiphase a.c. motor in accordance with digital control signals, a first regulation circuit regulates the frequency of the a.c. electrical signal applied to the motor and another regulation circuit regulates the voltage of the a.c. signal. Both regulation circuits are controlled by digital control signals in a series of dis- [52] 318/308; 318/227; 0 crete steps over a range of control values and the control signals are supplied periodically to the regulation [51] Int. Cl G05b 13/02; G05b 15/02 circuits The Speed and torque are controlled in GOP Fleld of Search dance an algorithm relating Speed and torque to the frequency and voltage of the source of electric [56] References cued power which is applied to the motor, calculated by UNITED STATES PATENTS computing apparatus on a periodic basis. In one em- 3,6l2,97l 10/1971 Blaschke 318/227 bodiment the position of the magnetic flux of the sta- 3,64l,408 2/1972 Fiocca 318/227 tor is controlled directly. 3,770,986 10/1972 Cushman 318/227 12 Claims, 38 Drawing Figures 2? /6 /2 SOFTWARE powgk com Mme osc/LumR POWER I4 u-a. (50-100 #2) RM/s/smk 7!?0/1/5/57'06 Comm/v0 Den/fies ans-1e .SCR Ac. .5 use/22 4702 emcee/v7 7'k/66Ek sctefs M ra/e P207667 c/zco/rs .36 .44 3o (sorrnaxz) 5c:

FIR/N6 F/k ING 2 ae/c M162: Alt GL5 AMPL/F/Ek man/9km? camM/vo can/7x02 Pan ER 73,878,445 SHEET cm 18 PAYENTESAPR 1 5 875 5 0 (FF me) PATENTEBAPR 1 51975 SHEET 150? 18 ."JENTEBAPR 1 51975 SHEET 17G! 18 000 0 00 0O0/// 000 0 /OO0// 0000/0 0// O0 O /OO/ OO //0 OO/ OO /00O aooo/ r rr r r A.C. MOTOR CONTROL APPARATUS AND METHOD BACKGROUND OF THE INVENTION This invention relates to apparatus for the control of induction motors and particularly to means for controlling the speed and torque of such motors in a way which permits rapid response to control signals when such motors are employed in servo systems.

In a servo system where the motor torque and speed characteristics are varied in accordance with a program or schedule, it is desirable to have a motor which responds quickly to programmed changes in its condition. It has been customary in the past to use d.c. motors for such applications. but d.c. motors have the major disadvantage of requiring brushes and a commutator assembly which adds to the complexity of the motor system and the amount of necessary maintenance. It is, therefore, desirable to employ a.c. motors, and particularly induction motors for this purpose. so thatthe simpler construction of the ac. motor can be used to advantage. However, when an induction motor is operated at less than its rated speed. a relatively high slip value results in increased heating within the motor. an undesirable condition. Such heating not only renders motor operation less efficient, but may also damage the motor.

In an effort to employ an ac. motor in a servo loop, it has been proposed to supply a variable frequency to the motor, so that the motor can operate over a variety of speeds at a substantially constant slip. It has also been proposed to vary the input voltage to the motor so that the output speed of the motor is a function primarily of the voltage input, and to combine both methods of control so that the amplitude of the driving voltage, and its frequency, are controlled together by some fixed relation. However, the means which have been developed in the past for doing this have not been entirely successful. The responses of the motor to commands for changing values of speed and torque have been relatively slow, and the apparatus employed has not permitted a'high degree of precision and accuracy in controlling the motor, nor any flexibility in the relation between frequency and voltage control.

One problem has been that with low frequency operation, there is a relatively long time between successive state changes of the windings which generate the stator field. In a three phase motor, for example, there are only six state changes per cycle, and the time interval between successive state changes, viz. one-sixth cycle of the rotor, becomes quite long at low rotor frequencies. Moreover, the flux which traverses the air gap between stator and rotor can be changed in value only relatively slowly, because of the high inductance of the rotor. These and other problems combine to suggest that the use of an induction motor is not feasible when precise control is required at low rotor speeds.

It is, therefore, a principal object of the present invention to provide a mechanism whereby precise control of the speed and torque of an induction motor is attained, even at very low speed conditions.

A further object of the invention is to provide for a flexible relation in controlling the frequency and voltage of the ac. power applied to the motor. 7

Another object of the present invention is to provide a mechanism for controlling an induction motor without causing a high degree of heating within the motor when operated at low speeds.

A further object of the present invention is to provide a mechanism whereby control of motor speed and torque are controlled by periodically recurring control signals which have a relatively high frequency in relation to the speed of the motor.

Another object of the invention is to provide digital apparatus for accepting periodic digital control signals from a numerical control apparatus and employing such signals to effect precise control of an induction motor.

A further object of the present invention is to provide apparatus and a method for generating periodic digital control signals in accordance with a specified algorithm relating to the speed of the motor and servo following error to the frequency and amplitude of the driving voltage applied thereto.

Another object of the present invention is to provide apparatus and a method for periodically calculating digital values for the frequency and amplitude of the driving voltage as a function of motor speed and following error, such function being derived empirically for the motor with which the apparatus is used.

A further object of the present invention is to provide apparatus and a method for directly controlling the position of the resultant magnetic flux generated in the stator of the motor as a function of the motor speed and servo following error.

Another object of the present invention is to provide apparatus for disconnecting the motor from its driving voltage in response to a condition of excessive motor current.

- A further object ofthe present invention is to provide apparatus and a method for modifying the number of poles in a multiphase a.c. motor, in response to a calculation involving motor speed and servo following error.

Another object of the present invention is to provide apparatus and a method for calculating the frequency of the driving voltage as a function of motor speed and servo following error, and selecting one of a series of discrete frequency values in response thereto, and applying a hysteresis quantity in said calculation for establishing one criterion for selecting one frequency, and a different criterion for selecting another frequency subsequent to selecting said one frequency.

These and other objects of the present invention will become manifest upon an examination of the following description and the accompanying drawings.

SUMMARY OF THE INVENTION In one illustrative embodiment of the present invention there is provided a plurality of power transistors connected to the several phases of a multiphase induction motor, a variable frequency voltage source connected to said transistors, and a variable voltage d.c. source selectively connected to said motor through said transistors in accordance with said variable frequency. Both the variable voltage source and the variable frequency source are controlled periodically by digital control signals. In connecting the variable frequency source to the motor, the power transistors maintain a connection between the voltage source and the several phases of the motor for varying periods of time in order to obtain precise control.

The motor current is sensed, and an excessive current condition quickly temporarily disables the connec- 

1. A servo system for controlling the application of electrical power to an a.c. motor, comprising means for generating a signal representative of the speed of said motor, frequency selecting means for selecting the frequency of an a.c. voltage to be applied to said motor, means for providing an input signal representative of a selected angular position for said motor, means for deriving a control signal in response to said input signal and to said speedrepresentative signal, and amplitude selecting means responsive to said control signal for selecting an amplitude for said a.c. voltage which amplitude is determined in accordance with a predetermined relation between the torque developed by said motor and the amplitude of an a.c. voltage applied thereto at said selected frequency, whereby the torque developed by said motor has a predetermined relation to said control signal, and means for connecting a source of electrical power to said motor, said last named means being connected with, and responsive to, said frequency selecting means and said amplitude selecting means.
 2. Apparatus according to claim 1, wherein Said frequency selecting means comprises means for selecting one of a plurality of fixed frequencies, in accordance with the speed of said motor, such that a lower frequency is used when the motor speed is lower than a predetermined value, and a higher frequency is used when the motor speed is higher than said predetermined value.
 3. Apparatus according to claim 1, wherein said amplitude selecting means comprises comparing means for comparing the value represented by said control signal with a fixed quantity, and means responsive to said comparing means for selecting an amplitude bearing one relation to said control signal when said value represented by said control signal is less than said fixed quantity, and bearing a second relation to said control signal when the value represented by said control signal is greater than said fixed quantity.
 4. Apparatus according to claim 1, wherein said frequency selecting means comprises means for comparing the value of said control signal with a fixed quantity, means for selecting a frequency having a first relation to said control signal when said control signal value is less than said fixed quantity, and means for selecting a frequency having a second relation to said control signal when said control signal value is greater than said fixed quantity.
 5. Apparatus according to claim 1, including switching means connected with said motor for alternately connecting said motor for operation with two different numbers of poles, and means responsive to said control signal and connected to said switching means for causing said motor to be connected for operation with a first set of poles in response to one range of values of said control signal, and for causing said motor to be connected for operation with a second set of poles in response to another range of values of said control signal.
 6. Apparatus according to claim 5, including means connected with said amplitude selecting means for selecting an amplitude of zero for a predetermined time interval during switching from one set of poles to the other set of poles.
 7. Apparatus according to claim 1, wherein said frequency selecting means comprises means for comparing the value of said control signal with one of first and second fixed values, means for changing the frequency selected for application to said motor from a lower frequency to a higher frequency when the value of said control signal exceeds the greater of said two values, and means for changing the frequency selected for application to said motor from said higher frequency to said lower frequency when the value of said control signal is less than the lesser of said two values.
 8. A servo system for controlling an a.c. motor for constant slip operation to drive a movable member in accordance with a specified program that designates the velocity and position of the movable member comprising means responsive to command signals representative of the designated instantaneous position of the movable member and to signals representative of the actual instantaneous position of the movable member for deriving a signal proportional to the torque required of the motor to follow the program, means for supplying an a.c. voltage to said motor, means responsive to said torque-representative signal for controlling the amplitude of said a.c. voltage, means for producing a speed signal proportional to the speed of operation of said motor, means for modifying said speed signal by increasing it by an amount corresponding to a predetermined constant slip, and means for controlling the frequency of the voltage applied to said motor in accordance with said modified speed signal.
 9. Apparatus according to claim 8, wherein the speedresponsive signal is modified by a quantity proportional to said amplitude, to give a slip proportional to said required torque.
 10. For use with a servo system for controlling the application of electrical power to an a.c. motor to regulate the movement of a movable member driven by said moTor, a method comprising the steps of generating a signal representative of the speed of the motor which will produce the desired velocity of said movable member, selecting the frequency of an a.c. voltage to be applied to said motor in response to the speed of the motor, deriving a control signal in response to a signal representative of a selected position of said movable member and to said speed signal, and selecting an amplitude for said a.c. voltage in response to said control signal which is in accordance with a predetermined relation between the torque developed by said motor and the amplitude of an a.c. voltage applied thereto at said selected frequency.
 11. For use with a servo system for controlling the application of electrical power to an a.c. motor, a method comprising the steps of deriving a torque-representative signal proportional to the torque required of the motor to follow a predetermined program, controlling the amplitude of an a.c. voltage applied to said motor in response to said torque-representative signal, producing a speed signal proportional to the speed of operation of said motor, modifying said speed signal by increasing it by an amount corresponding to a predetermined constant slip, and controlling the frequency of the voltage applied to said motor in accordance with said modified speed signal.
 12. The method of controlling the operation of an a.c. motor in accordance with signals from a record which represent a program of movements of a movable member driven by said motor, comprising the steps of generating a signal representative of the speed of the motor in response to information from said record designating the desired velocity of said movable member, selecting the frequency of an a.c. voltage to be applied to said motor in response to the speed of the motor, deriving a control signal in response to the distance of movement of said movable member during a measured period of time and to said speed signal, and selecting an amplitude for said a.c. voltage in accordance with said control signal to establish the desired operation of said motor for actuating the movable member in accordance with the program on said record. 